Amplitude modulators



June 11, I957 E. c. DENCH 2,795,760

AuPLxiunE MODULATORS Filed 30, 1953 3 Sheets-Sheet 1 SOLENOID ,PRODUCINGAXIAL MAGNET/C FIELD 7H? SUPPL IMODULAT'OR H54 Y F76. 4 Mow /NVENTORLAT/ON nvpur SIGNAL EDWARD C. DENCH June 11, 1957 E C, BENCH 2,795,760

AMPLITUDE MODULATORS Filed Jan. 30, 1953 3 Sheets-Sheet 2 MODULATION IIVU T SIGNAL & l6

F'IG. 5 MODULATOR 6mm LOAD MODULATION INPUT SIGNAL FIG. 6 MODULATOR 2 /0ofl 30 I 3/ I VAR/ABLE VARIABLE GENERATOR "IRANSERMER WANiEg/ZMER LOAD12; Y 1 In z [-713 7 GENERATOR /oa D i o# I l' "i l VARIABLE us VAR/ABLE-/4 PULL -4 TRANSFORMER MODULATOR TRANSFORMER MODJLAf/aM' //VPUTSIIGA/AL 27 {L 38a V 7 37a 3 INVENTOI? EDWARD C. DENCH June 11,1957 E.C. DENCH 2,795,760

AMPLITUDE HODULATORS Filed Jan. so, 1953 z sheets-sheet :s

RECEIVER 1 I VARIABLE PUSH VARIABLE -/4 puu. 4 mum/2mm MODULATORTRANSFORMER Mt -A770 l/VWTSEAML INVENTOR EDWARD C. DENCH CZVM- A TORNEYUnited States Patent AMPLITUDE MODULATORS Edward C. Dench,Needham,Mass., assignor to Raytheon Manufacturing Company, Newton,Mass., a corporation of Delaware Application January 30, 1953, SerialNo. 334,262

11 Claims. (Cl. 332-) This application relates to an amplitudemodulating device and, more particularly, relates to a system foramplitude modulating a signal generator by means of an electricallycontrollable quarter-wave transformer.

Cylindrical magnetron diodes having an anode and cathode coaxially:arranged with respect to one another and having a magnetic fieldtransverse to the electrical field between anode and cathode are wellknown. If a source of relatively low voltage is connected between theanode and cathode, a space charge cloud forms about the cathode whoseradius is dependent upon the anode voltage E2. and the magnetic field B.

If the magnetic field B is such that the dielectric constant of thespace charge is negative, the space charge cloud behaves as though itwere a solid cylindrical conductor serving as the inner conductor of acoaxial line. The radius r, of the virtual inner conductor may be variedover a considerable range in response to the applied magnetron anodevoltage. Since the anode voltage is preferably maintained below acritical voltage at which anode current becomes appreciable, the maximumradius mlx of the virtual inner conductor is limited to a valuecorresponding to said critical anode voltage. Inasmuch as thecharacteristic impedance of a coaxial line is a function of the ratio ofthe radii of the outer and inner conductors of the line, a variation incharacteristic impedance of this variable section of coaxial line isachieved by variation of the anode voltage.

If the variable section of coaxial line is made long where n is anyinteger including zero and A is the nominal operating wave lengthof thesystem with which the variable section is to be used, this coaxial linesection becomes an electrically variable quarter-wave transformer. Byinserting this quarter-wave transformer whose iterative impedance may bevaried between the source of electromagnetic energy, such as amagnetron,

charge cloud about the cathode of a coaxial type magnetron diode;

Fig. 2 is a curve illustrating certain characteristics of the device ofFig. 1;

- circular.

and a load, such as an antenna, the impedance matching between thesource and loadand hence the load power variation-may be varied inaccordance with the aforesaid anode voltage.

If greater modulation is required, two of such variable transformers maybe placed-in series between the Fig. 1 indicates the distribution of theelectron space i Fig. 3 illustrates certain details of a quarter-wavetransformer with pertinent dimensions shown;

Fig. 4 illustrates a fragmentary cross-sectional view of a quarter-wavetransformer showing certain structural details omitted for the sake ofclarity from Fig. 3;

Fig. 5 is a block diagram of a first embodiment of an amplitudemodulation system utilizing the transformer shown in Figs. 3 and 4;

Fig. 6 is a block diagram of a first modification of the amplitudemodulation system] shown in Fig. 5;

Fig. 7 is a block diagram of a second embodiment of an amplitudemodulation system; and i i Fig. 8 is a block diagram of a thirdembodiment of an amplitude modulation system.

Fig. 1 indicates a typical cross section of a coaxial cylindricalmagnetron 10 having an electron-emitting cathode 11 of diameter r and ananode 12 of'diameter r spaced concentrically therefrom. For purposes ofexplanation, the cathode may be either hollow or solid. A magnet (notshown in Fig. 1) also forms part of a magnetron and.

produces a strong magnetic field normal to the region between cathodeand anode. If a source of voltage Ea of the proper magnitude isconnected between cathode and anode, so that the anode is positive withrespect to the cathode, a cloud of electrons 14 will form about thecathode; the outer boundary 15 of this cloud is substantially If themagnitude of the anode voltage is increased, the radius r of the spacecharge cloud likewise increases. V

Collisions occur between electrons in the space charge 12. Because ofthese collisions an energy exchange between electrons occurs so thatsome electrons gain energy while others lose energy. Those electronswhich gain energy will bombard and heat the cathode. Those'electronswhich lose energy can escape from the space charge cloud and will befound at a radius greater than r In order to heat the cathode bybombardment, the power must come from the anode supply and, since, for asmall anode voltage, no anode current is assumed, then for small anodevoltages the energy exchangemust be such that no cathode bombardmentoccurs;

At the onset of this cathode bombardment effect, electrons will bereleased from the cloud] A necessary condition is that these electronsmust reach the anode. Since, at the onset of bombardment, the energy ofbombardment is quite small, the released electrons fromthe virtualcathode of radius r are small in number, the potential distribution ofthe space between r and Ta is not affected by their presence andequations assuming a charge-free space in this region are still valid.

In order to determine thegcritical anode voltage at which bombardmentbegins, it is'necesary to solve-the Hull cutoff condition for amagnetron having a virtual cathode of radius r and an anode ofradius-rs.

If E is the voltage existing at the edge of the electron cloud, the Hullcutoff equation becomes where tion between E and E in terms of theparameters r and r Since both Equations 1 and 2 must be satisfiedsimultaneously at the onset of bombardment, they can be equated to yieldthe following equation implicit in From Equation 3 it is evident thatfor every value of there is a critical value of 0 corresponding tovalues of "h are shown in Fig. 2.

Referring to Figs. 3 and 4, the variable transformer section 10comprises a coaxial magnetron diode having a cylindrical inner conductoror cathode 11 and a cylindrical outer conductor or anode 12. As shown inFig. 4, one end of transformer 10 is connected to a generator and theother end to aload. The inner conductor 11 has a reduced portion 11which is made a quarter-wave length long for the nominal frequency ofoperation of the system in which the transformer is to be used. Thetransformer may, of course, be made any oddnumber of quarter-wavelengths long. An axial'magnetic field is provided by means of anelectromagnet 17 surrounding the anode; it is possible to use apermanent magnet in lieu of an electromagnet where variation of fieldstrength is not required.

A modulator or source of variable voltage 16 is connected between theinner and outer conductors as schematically shown in Fig. 4. As theoutput of modulator 16 varies, the radius of the space charge cloud 14.varies over the range from r to Although a system for varying thecathode-anode voltage of electron discharge device 10 has beendescribed, it is also possible to vary the dimensions of the space.charge cloud'by varying the strength of the magnetic field, as byvarying the current flowing in the electromagnet 17. The difference inradii of theportions 11 and 11' of the inner conductor is preferablysuch that the maximum outer boundary of. the space charge cloudcoincides with the normal portion 11. Since r is equal to or less thanmax the construction provides its own end shields to minimize cathodeemission inthe longitudinal direction.

A cathode heater element 18 is inserted within the reduced portion 11'of the hollow inner conductor and heater leads 19, 19 may be brought outthrough beaded apertures 22 in the coaxial assembly for connection to asource of heater voltage. The magnetron is vacuum sealedby means ofdisks 20 and 21 which may be joined to-the inner and outer conductors byconventional disksealing techniques.

In Fig. 5 a typical circuit for amplitude modulation is shown whichcomprises a generator 25 of electromagnetic energy, such as a magnetron,a load 27, such as an antenna, a quarter-wave transformer 10 of the typeshown in Figs. 3 and 4 and a modulator 16.

Generator 25 is connected to the quarter-wave transformer 10 by means ofa coaxial transmission line 30 whose length is where n is any integer. Aline of this length acts as a 1-to-1 transformer and insures that theimpedance at the generator will be the same as that at the junction ofline 30 and the quarter-wave transformer 10. The other end oftransformer 10 is connected to load 27 by means of a transmission line32.

As is well known the characteristic impedance Z of a coaxial line isgiven by 138 D ia- 810 d where D is the diameter of the outer conductor,d is the diameter of the inner conductor and e is the dielectricconstant of the dielectric used in the line.

In terms of radii r and r this equation, assuming air as the dielectric,becomes Z 138 logw For purposes of explanation, the ratio'of the anoderadius r to the cathode radius r will be assumed to be 3. Referring toFig. 2, the ratio max corresponding to a ratio of 3 is found to be 1.56.The space charge cloud may thus be varied from without anode currentflowing. The iterative impedance of transformer 10 may be determinedfrom Equation 5;

it is seen to vary from 26.6 to 65.8. The input impedance Z5 is given byz. ZR,

26.6 -26.6 ohms and, neglecting losses in the line, all the energyproduced by generator 25 will be transferredto the load. If

the inputimpedance' Zs of" transformer 10 from Equation 6 bec'omes Inother words, the input impedance :of transformer 10 for a load of 26.6ohms can be varied from 26.6 ohms to =162.5 ohms (7) 162.5 ohms. If themagnetron operates at constant voltage the load power variation is givenby Since the power is a function of the square of the source voltage,the load voltage variation achieved is or 2.47-to-1 The percentagemodulation is given by Average envelope amplitudeminimum envelopeamplitude Percent m0d Average envelope amplitude X :3)

For a voltage variation of 2.47 the voltage'modulation is length where nis any integer and 7\ is the wavelength at the operating frequency ofthe system, so that the coaxial line between the two transformers has noeffect upon the impedance represented by the serially connectedtransformers.

The modulating voltage from modulator 16 is applied in push-pull to theindividual transformers so that, when the characteristic impedance ofone transformer is 26.6, the characteristic impedance of the other is65.8 and vice versa. Assume that transformer 10a has a characteristicimpedance of 65.8 at the time when the modulator voltage applied to itis zero while at the same time the transformer 10b has a characteristicimpedance of 26.6 (when the modulator voltage applied to it is maximum).Assume also that the load is 26.6 ohms as before. The impedance of theload is matched to that of transformer 10b and the input impedance ofthe latter is also 26.6. This impedance is the same as the output orload impedance of transformer 10a, since transformers 10a and 10b areseparated by an integral number of half-wave lengths. The inputimpedance to transformer 10a then becomes The input impedance totransformer 10a then becomes It is evident, therefore, that a variationof inputirnpedance of 162.5 to 4.36 is obtained.

and the voltage ratio is or 6.1-to-1 this corresponds to a modulation ofseventy-two percent.

In Fig. 7, a system is shown for utilizing a pair of quarter-wavetransformers for shifting energy from one load to another, as inelectronic scanning.

A generator 25 is connected by appropriate transmission means to twoquarter-wave transformers 10a and 10b. For example, generator 25 maycomprise a magnetron oscillator with a conventional coaxial outputcoupling; a coaxial line section can then be connected between theoutput coupling and the corresponding coaxial quarterwave transformer10a or 10b. Energy transferring means are well known in the art and neednot be described in detail.

The output of a push-pull modulator circuit 16 is connected to the twotransformers 10a and 10b. By pushpull control of these transformers,power from generator 25 may be continuously and smoothly transferredfrom one load to the other. As shown in Fig. 7, the tWo loads compriseantennas 27a and 27b, respectively, including radiating elements 37a and37b located at the focal points of corresponding parabolic reflectors38a and 3812. As the impedances of the transformers 10a and 10b changewith varying output of modulator 16, the energy radiated fromeachantenna cyclically changes. At one instant the radiation fromantenna 371: is a maximum while that from antenna 37b is a minimum;degrees later in the modulating cycle, the radiation from antenna 37a isa minimum while that from antenna 37b is a maximum. In both these twoconditions the energy radiated from the two antennas is continuouslyvarying. The system of Fig. 7 can thus be used in radar scanningsystems.

The system of Fig. 7 may be modified for reception of energy as shown inFig. 8.

The system of Fig. 8 comprises receiving antennas 27a and 27b,respectively, including receiving elements 37a and 37b located at thefocal points of corresponding refiectors 38a and 38b. It is possible, ofcourse, to use any type of antenna other than the type shown in Fig. 8for reception.

The signals received by antennas 27a and 27b are fed to variablequarter-wave length transformers 10a and 10b, respectively, which areinserted between the antennas and a receiver 45. A push-pull modulatorcircuit 16 feeds each of the aforesaid transformers thereby varying theimpedances of said transformers in the same manner as the system of Fig.7. In this way, the amount of energy received by receiver 45 iscyclically varied.

This invention is not limited to the particular details of construction,materials and processes described, as many equivalents will suggestthemselves to those skilled in the art. It is accordingly desired thatthe appended claims be given a broad interpretation commensurate withthe sgr pe of the invention within the art.

What is claimed is:

1. In combination: an input transmission line and an output transmissionline each having coaxially positioned inner and outer conductors and acoaxial line impedance matching transformer connected between said inputand output transmission lines; said transformer comprising an electrondischarge device having an anode whose diameter is equal to the diameterof said outer conductor of said transmission lines, a cathodeconcentrically arranged with respect to said anode and having a diametersmaller than that of said inner conductor of said transmission lines,the ends of said cathode terminating in end shields positionedsubstantially perpendicular to said cathode, said cathode being any oddnumber of quarter-wave lengths long at the desired operating frequency,a source of potential connected between said'anode and cathode forproducing an electrical field therebetween, and means for producing amagnetic field normal to said electrical field; said electrical andmagnetic fields interacting to produce an electron space charge cloudabout said cathode whose radius is a function of the strength of saidinteracting fields, and control means for varying said fields to varythe impedance match between said input and output transmission lines.

2. In combination: an input transmission line and an output transmissionline each having coaxially positioned inner and outer conductors and acoaxial line impedance matching transformer connected between said inputand output transmission lines; said transformer comprising an electrondischarge device having an anode whose diameter is equal to the diameterof said outer conductor of said transmission lines, a cathodeconcentrically arranged with respect to said anode and having a diametersmaller than that of said inner conductor of said transmission lines,the ends of said cathode terminating in end shields positionedsubstantially perpendicular to said cathode, said cathode being any oddnumber of quarter-wave lengths long at the desired operating frequency,a source of potential connected between said anode and cathode forproducing an electrical field therebetween, and means for producing amagnetic field normal to said electrical field; said electrical andmagnetic fields interacting to produce an electron space charge cloudabout said cathode whose radius is a function of said potential, andcontrol means for varying said potential and thereby the impedance matchbetween said input and output transmission lines.

3. In combination: a generator of electromagnetic energy, a load, atransmission line having .coaxially positioned inner and outerconductors interconnecting said generator and said load and a coaxialline impedance matching transformer inserted in said transmission line;said transformer comprising an electron discharge device having an anodewhose diameter is equal to the diameter of said outer conductor of saidtransmission line, a cathode concentrically arranged with respect tosaid anode and having a diameter smaller than that of said innerconductor of said transmission line, said cathode having a length wheren is any odd integer and A is the operating wave length of saidgenerator, a source of potential connected between .said anode andcathode for producing an electrical field therebetween, and means forproducing a magnetic field normal to said electrical field; saidelectrical and magnetic fields interacting to produce an electron spacecharge cloud about said cathode whose radius is a function of saidpotential, and control means for varying said potential and thereby theamount of energy transferred from said generator to said-load.

4. In combination: a generator of electromagnetic energy, a load, -atransmission line having coaxially positioned inner and outerconductorsand interconnecting said generator and said load, and a pairof serially connected impedance matching transformers inserted in saidtransmission :line; said transformers each comprising a magnetron havingan anode whose diameter is equal to the diameter of saidouter conductorof said transmission line, a cathode concentrically arranged withrespect to said anode and having a diameter smaller than that of saidinner conductor of said transmission line, said cathode having a lengthwhere n is any odd integer and 7t is the operating wave length of saidgenerator, a source of potential connected between said anode andcathode for producing an electrical field therebetween, and means forproducing a magnetic field normal to said electrical field; saidelectrical and magnetic fields interacting to produce an electron spacecharge cloud about said cathode and whose radius is a function of thestrength of said interacting fields, said source of potential includinga push-pull modulator whose output circuit is connected to each of saidtransformers and whose input circuit is supplied with a modulation inputsignal for varying said interacting fields and thereby the amount ofenergy transferred from said generator to said load. V

5. In combination: a generator of electromagnetic energy, a load, atransmission line having coaxially positioned inner and outer conductorsand interconnecting said generator and said load, and a pair of seriallyconnected impedance matching transformers inserted in said transmissionline; said transformers each comprising a magnetron having an anodewhose diameter is equal to the diameter of said outer conductor of saidtransmission line, a cathode concentrically arranged with respect tosaid anode and having a diameter smaller than that of said innerconductor of said transmission line, said cathode having a length wheren is any odd integer and 7\ is the operating wave length of saidgenerator, a source of potential connected between said anode andcathode for producing an electrical field therebetween, and means forproducing a magnetic field normal to said electrical field; saidelectrical and magnetic fields interacting to produce an electron spacecharge cloud about said cathode whose radius is a functionof saidpotential, said source of potential including a push-pull modulatorwhose output circuit is connected to each of said transformers and whoseinput circuit is supplied with a modulation input signals for varyingsaid PQtential and thereby the amount of energy transferred from saidgenerator to said load.

6. In combination: a generator of electromagnetic energy, a pair ofloads, a transmission line having coaxially positioned inner and outerconductors and interconnecting said generator and each of said loads,and an impedance matching transformer inserted in each of saidtransmission lines; said transformers each comprising an electrondischarge device having an anode whose diameter is equal -to thediameter of said outer conductor of said transmission line, a cathodeconcentrically arranged with respect to said anode and having a diametersmaller than that of-said inner conductorof said transmission line, saidcathode being an odd numberof quarter-wave lengths long at thedesiredoperating frequency, a source of potential connected between said anodeand cathode for producing an electrical field therebetween, and meansfor producing a magnetic field normal to said electrical field; saidelectrical and magnetic fields interacting to produce an electron spacecharge cloud about said cathode whose radius is a function of themagnitude of said interacting fields, said source of potential includinga push-pull modulator whose output circuit is connected to each of saidtransformers and whose input circuit is supplied with a modulation inputsignal for varying the magnitude of said fields and thereby the amountof energy transferred from said generator to each of saidloads.

7. In combination: a generator of electromagnetic energy, a pair ofloads, a-transmission line having a coaxially positioned inner and,outer conductors and interconnecting said generator and each of saidloads, and an impedance matching transformer inserted in each of saidtransmission-lines; said transformers each comprising an electrondischarge device having an anode whose diameter is equal to the diameterof said outer conductor of said transmission line, a cathodeconcentrically arranged with respect to said anode and having a diametersmaller than that of said inner conductor of said transmission line,said cathode being an odd number of quarter-wave lengths long at thedesired operating frequency, a source of potential connected betweensaid anode and cathode for producing an electrical field therebetween,and means i for producing a magnetic field normal to said electricalfield; said electrical and magnetic fields interacting to produce anelectron space charge cloud about said cathode whose radius is afunction of said potential, said source of potential including apush-pull modulator whose output circuit is connected to each of saidtransformers and whose input circuit is supplied with a modulation inputsignal for varying said potential and thereby the amount of energytransferred from said generator to each of said loads.

8. In combination: a pair of receiving antennas, a receiver, atransmission line having coaxially positioned inner and outer conductorsand interconnecting each of said antennas and said receiver, and animpedance matching transformer inserted in each of said transmissionlines; said transformers each comprising an electron discharge devicehaving an anode Whose diameter is equal to the diameter of said outerconductor of said transmission line, a cathode concentrically arrangedwith respect to said anode and having a diameter smaller than that ofsaid inner conductor of said transmission line, said cathode being anodd number of quarter-Wave lengths long at the desired operatingfrequency, a source of potential connected between said anode andcathode for producing an electrical field therebetween, and means forproducing a magnetic field normal to said electrical field; saidelectrical and magnetic fields interacting to produce an electron spacecharge cloud about said cathode whose radius is a function of themagnitude of said interacting fields, said source of potential includinga push-pull modulator whose output circuit is connected to each of saidtransformers and Whose input circuit is supplied with a modulation inputsignal for varying the magnitude of said fields and thereby the amountof energy transferred from each of said generators to said receiver.

9. In combination, a transmission line having concentrically positionedinner and outer conductors for transmitting energy from an energy sourceto a load and a coaxial line impedance matching transformer inserted insaid transmission line, said transformer comprising an electrondischarge device having an anode and a cathode concentrically arrangedwith respect to one another and with respect to said conductors, saidcathode and said inner conductor constituting a single continuousmember, said cathode being any odd number of quarter wave lengths longat the desired operating frequency, a source of potential connectedbetween said anode and cathode for producing an electric fieldtherebetween, and means for producing a magnetic field normal to saidelectric field; said electric and magnetic fields interacting to producean electron space charge cloud about said cathode whose radius is afunction of the magnitude of said potential, said cathode having aradius smaller than that of 10 said inner conductor, and control meansfor varying said potential to alter the amount of energy transferred bysaid line.

10. In combination, a transmission line having concentrically positionedinner and outer conductors for conductor by means of portions extendingsubstantially perpendicularly to said cathode, said cathode being anyodd number of quarter wave lengths long at the desired operatingfrequency, a source of potential connected between said anode andcathode for producing an electric field therebetween, and means forproducing a magnetic field normal to said electric field; said electricand magnetic fields interacting to produce an electron space chargecloud about said cathode whose radius is a function of the magnitude ofsaid potential, and control means for varying said potential to alterthe amount of energy transferred by said line.

11. In combination, a transmission line having concentrically positionedinner and outer conductors for transmitting energy from an energy sourceto a load and a coaxial line impedance matching transformer inserted insaid transmission line, said transformer comprising an electrondischarge device having an anode and a cathode concentrically arrangedwith respect to one another and with respect to said inner and outerconductors, said cathode being joined at both ends thereof to said innerconductor by means of portions extending substantially perpendicularlyto said cathode, said cathode being any odd number of quarter Wavelengths long at the desired operating frequency, a source of potentialconnected between said anode and cathode for producing an electric fieldtherebetween, and means for producing a magnetic field normal to saidelectric field; said electric and magnetic fields interacting to producean electron space charge cloud about said cathode whose radius is afunction of the magnitude of said potential, said cathode having aradius smaller than that of said inner conductor by an amount at leastequal to the radial distance between the outer periphery of said cathodeand the outer boundary of said electron space charge cloud correspondingto a critical value of said potential at which substantial anodecur-rent just begins to flow, and control means for varying saidpotential to alter the amount of energy transferred.

References Cited in the file of this patent UNITED STATES PATENTS2,153,728 Southworth Apr. 11, 1939 2,241,976 Blewett et a1. May 13, 19412,402,184 Samuel June 18, 1946 2,438,367 Keister Mar. 23, 1948 2,602,908Linder July 8, 1952

